1. Field of the Invention
This invention relates to a rear converter mounted on the image side of an objective lens to enlarge the composite focal length, and in particular to a rear converter mounted on a telecentric objective lens to maintain the telecentricity of the emergent light beam.
2. Description of the Prior Art
A conversion lens is used chiefly to change the focal length range of an objective lens. A conversion lens mounted on the front side of an objective lens to shift the focal length range to the long focus side is called a front teleconverter, and a conversion lens mounted on the rear side of an objective lens is called a rear teleconverter. These two systems have their own merits and demerits, and the rear converter is often used because it can be made very compact and light-weight. The principle of a popular rear converter, as shown in FIG. 1 of the accompanying drawings. is that a converter C as a lens system having a negative power is added to the rear side of an objective lens L and is caused to function to convert the focal length as a whole into the long focal length side and maintain the imaging performance. In that case, it is usual that the inclination of the principal ray of the oblique light beam having passed through the objective lens L is further inclined by the action of the negative power of the converter C after the light beam passes through the converter C, and the exit pupil comes nearer to the image side.
In a camera which records images by means of an ordinary silver salt photosensitive material, even if this principal ray of the oblique light beam enters the photosensitive surface while being considerably inclined, it does not affect the quality of image and therefore, it is rather usual to cause the principal ray to enter while being inclined for the purpose of making the optical system compact or making the portion connected to the camera body compact. However, in case images are recorded by means of an image pickup tube or element for video, there are a low-pass filter, a stripe filter, etc. before the light beam passes through the lens reaches the photoelectric surface, and particularly, the spacing between the stripe filter and the photoelectric surface has a thickness of the order of 20-50.mu. and therefore, if the light beam enters obliquely, it may cause poor synchronization of electrical signals and unclear images. Particularly, in the case of a color stripe filter used in a color video camera, if the light beam enters with an inclination, it will sensitize a location deviated from a predetermined photosensitive zone after having passed through the color stripe filter and may cause occurrence of color misregistration and aggravation of the quality of image. Therefore, generally, in the optical system for a video camera, there is adopted an optical system through which the principal ray of the oblique light beam enters the photosensitive surface nearly perpendicularly thereto, that is, a telecentric optical system.
Again in the telecentric optical system used in such a lens for a video camera, if there is a rear converter, the application thereof will conveniently become wider. For example, if a rear converter of 2 magnifications is mounted on a zoom lens of 10 zoom ratio (f=10-100 mm), the focal length will become 20 mm-200 mm and will generally cover a focal length range twenty times, and this will mean a tremendous power. Moreover, the size of the converter itself is very small and handy as compared with the body lens. However, where a rear converter is used for video, there is another problem. The rear converter which adds a negative power to the rear side of the objective lens as previously described displaces the incident light beam as shown in FIG. 1 and thus deviates from the telecentricity which is important for the optical system for video. This is considered to be a matter of basic principle, and a rear converter which changes the focal length while maintaining its telecentricity has been regarded as impossible. Thus, heretofore, it has been given up to mount a rear converter on a telecentric optical system for video and large front converters have been used, or the deviation from the telecentricity has been regarded as inevitable and small rear converters have been used with a result that aggravation of images and occurrence of color misregistation have been unavoidably tolerated.